Avise's Birds of the World

Pacific-slope Flycatcher
Empidonax difficilis

Native Range: North America

Notes: this is one in a large group of small-sized Empidonax flycatchers that constitute a "sibling-species" complex, meaning that the species are nearly impossible to tell apart from one another in the field by plumage and morphology alone; instead, identification to a particular species relies on a combination of clues including habitat, subtle plumage features, behaviors in some cases, and especially the species' distinctive songs (which originally gave biologists the bes thint that multiple species were present).